How many shots per minute can be fired by a muzzle-loading gun?

All depends on the type. Smoothbore muzzle loaders (such as the Brown Bess) were a bit faster than rifled weapons of the period (such as the Kentucky rifle), and a user on average could get off three shots with the Brown Bess, vs. one with the Kentucky, in a minute's time.

With the advent of percussion caps, rifled weapons were able to attain a typical two to three rounds per minute.